Supporters of the move say it is a widespread myth that cohabiting couples enjoy the same rights as if they had married. There are almost three million cohabiting couples in Britain.
The Law Commission suggested six years ago that former partners who have lived together for two years should be able to make a financial claim if they break up.
The amount awarded could reflect whether claimants suffered a financial loss – by giving up a job, for example – as a result of the relationship.
Lib Dem activists, tacitly supported by the party leadership, are calling for the proposals to be implemented.
They are also pressing for the laws on intestacy to be reformed to protect cohabitants whose partner dies without leaving a will.
Critics will claim the plans risk undermining the status of marriage. But a recent poll of MPs discovered there was cross-party support for the changes, with reform supported by 60 per cent of parliamentarians.
The vote will be held on Monday 23rd September 2013 and, if passed, will become official Lib Dem policy.